Picosecond Time-resolved Absorption Research Articles

Primary photophysics of the FMN binding LOV2 domain of the plant blue light receptor phototropin of Avena sativa

Abstract The temporal evolution of the initially excited singlet state of flavine mononucleotide, which is the cofactor in the LOV2 domain of the blue photoreceptor phototropin, has been studied in picosecond time-resolved fluorescence and femtosecond time-resolved absorption experiments. In the LOV2-WT protein of Avena sativa singlet–triplet intersystem crossing proceeding within 2.3 ns is the primary process which increases the triplet yield by a factor of 1.23 as compared to a mutant where cysteine 39 is replaced by alanine. This flavin triplet state is responsible for the formation of a cysteinyl–flavin adduct which triggers the unique photocycle of the LOV2 domain and thus the sensoric function of the blue light receptor phototropin.

Picosecond transient absorption spectroscopy of trans-1-(9-anthryl)-2-(4-R-phenyl)ethylenes with electron-donating and -accepting substituents

The spectroscopic properties of the title compound styrylanthracene (StA) and its derivatives (R: NO 2, CN, Br, Me, OMe and NMe 2) were studied by picosecond time-resolved absorption techniques in solution at room temperature. The lifetime of the excited singlet state S 1 varies with increasing the solvent polarity from 1–3 ns to ⩽100 ps for compounds with strong electron-donating (OMe and NMe 2) and -accepting (CN and NO 2) groups. The effect of polarity on the deactivation pathways of these push–pull polyenes is suggested to be due to an intramolecular charge transfer.

Excited States of 4-Aminobenzonitrile (ABN) and 4-Dimethylaminobenzonitrile (DMABN): Time-resolved Resonance Raman, Transient Absorption, Fluorescence, and ab Initio Calculations

Structures of the locally excited (LE) states of 4-aminobenzonitrile (ABN) and 4-dimethylaminobenzonitrile (DMABN) have been studied by picosecond time-resolved resonance Raman and transient absorption spectroscopy. Contrary to reported time-resolved infrared experiments, our time-resolved resonance Raman spectra show characteristic frequency shifts of several modes from their ground-state values. Combined with ab initio geometric and vibrational analysis of the ground and LE states, our results indicate a similar planar structure for the LE states of both ABN and DMABN. In the LE state, the pyramidal conformation of the ground state flattens along the inversion coordinate, the phenyl ring expands, and the ph-N bond shortens. Partial charge transfer in the LE state, mainly from the amino nitrogen lone pair orbital to the ring π* orbital, is indicated by the observation of a frequency downshift of the ring CC stretching mode and an ∼30 cm-1 downshift of the C⋮N stretching mode. This is supported by calculated changes of electron distribution in molecular orbitals upon excitation.

Modified windmill porphyrin arrays: coupled light-harvesting and charge separattion, conformational relaxation in the S1 state, and S2-S2 energy transfer.

The architecture of windmill hexameric zinc(II) -porphyrin array 1 is attractive as a light-harvesting functional unit in view of its three-dimensionally extended geometry that is favorable for a large cross-section of incident light as well as for a suitable energy gradient from the peripheral porphyrins to the meso-meso-linked diporphyrin core. Three core-modified windmill porphyrin arrays 2-4 were prepared for the purpose of enhancing the intramolecular energy-transfer rate and coupling these arrays with a charge-separation functional unit. Bisphenylethynylation at the meso and meso' positions of the diporphyrin core indeed resulted in a remarkable enhancement in the intramolecular S1-S1 energy transfer in 2 with tau=2 approximately 3 ps, as revealed by femtosecond time-resolved transient absorption spectroscopy. The fluorescence lifetime of the S2 state of the peripheral porphyrin energy donor determined by the fluorescence up-conversion method was 68 fs, and thus considerably shorter than that of the reference monomer (150 fs), suggesting the presence of the intramolecular energy-transfer channel in the S2 state manifold. Such a rapid energy transfer can be understood in terms of large Coulombic interactions associated with the strong Soret transitions of the donor and acceptor. Picosecond time-resolved fluorescence spectra and transient absorption spectra revealed conformational relaxation of the S1 state of the diporphyrin core with tau = 25 ps. Upon photoexcitation of models 3 and 4, which bear a naphthalenetetracarboxylic diimide or a meso-nitrated free-base porphyrin attached to the modified diporphyrin core as an electron acceptor, a series of photochemical processes proceeded, such as the collection of the excitation energy at the diporphyrin core, the electron transfer from the S1 state of the diporphyrin to the electron acceptor, and the electron transfer from the peripheral porphyrins to the diporphyrin cation radical, which are coupled to provide a fully charge-separated state such as that in the natural photosynthetic reaction center. The overall quantum yield for the full charge separation is better in 4 than in 3 owing to the slower charge recombination associated with smaller reorganization energy of the porphyrin acceptor.

Characterization of bisazo compounds employing ultrafast spectroscopy

Fluorenone-based bisazo compounds with different substituents have been investigated in various organic solvents and in a solid state by means of absorption, fluorescence and picosecond time-resolved absorption pump–probe spectroscopy. The bisazo compounds were found to coexist in azo–enol and hydrazone–quinone (HQ) tautomeric forms. Absorption spectra of the two forms were determined and excited-state properties were characterized. The azo–enol tautomeric form has a short, of tens of ps, excited-state lifetime and shows strong excited-state absorption, whereas stimulated emission is weak or absent. The HQ tautomeric form has a longer excited-state lifetime and shows sizeable stimulated emission. The main spectroscopic features of bisazo molecules in a solid state are similar to those in solutions, however, the excited-state relaxation is influenced by the specific solid phase effects. The lifetimes of the HQ form molecules in solutions differ by more than 10 times for bisazo compounds with different substituents and correlate with the photosensitivity of xerographic layers based on these compounds.

Solvent effects on the photophysical and photochemical properties of (E,E,E )-1,6-bis(4-nitrophenyl)hexa-1,3,5-triene

The photophysical and photochemical properties of (E,E,E)-1,6-bis(4-nitrophenyl)hexa-1,3,5-triene [(E,E,E)-1] have been studied in various solvents. The fluorescence emission maxima of (E,E,E)-1 show an increasing Stokes shift with increasing solvent polarity. Picosecond time-resolved fluorescence (TF) and transient absorption (TA) spectra do not show any significant time-dependent shifts in nonpolar solvent whereas, in more polar solvents, large red and blue shifts are observed in the TF and TA spectra, respectively. The fluorescence quantum yield reaches a maximum in moderately polar solvents and the quantum yield of intersystem crossing decreases strongly with increasing solvent polarity. Z–E-Isomerization of triene double bonds is inefficient in all solvents. In contrast, the absorption and fluorescence maxima for (E,E,E)-1,6-bis(4-cyanophenyl)hexa-1,3,5-triene [(E,E,E)-2] are practically solvent-independent. When the solvent polarity is increased, fluorescence quantum yield decreases monotonically and Z–E-isomerization quantum yield increases strongly. The results for (E,E,E)-1 can be understood in terms of an additional charge transfer excited state, which is absent for (E,E,E)-2.

Picosecond Time-Resolved Infrared Absorption Studies on the Photoexcited States of Poly(p-phenylenevinylene)

Picosecond time-resolved infrared absorption spectra of the photoexcited states of poly(p-phenylenevinylene) [(C6H4CHCH)n, PPV] have been observed with tunable picosecond light pulses obtained by difference-frequency generation between signal and idler waves from an optical parametric generator and amplifier. The time-resolved photoinduced infrared absorption bands due to a vibrational transition observed at 1550 cm-1 and an electronic transition at 3000 cm-1 of the photoexcited PPV have fast (≈1.7 ps) and slow (≈50 ps) decay components. By comparison of the picosecond time-resolved infrared absorption spectra of short-lived (fast decay component) and long-lived (slow decay component) transient species with the photoinduced and doping-induced infrared difference spectra of PPV, the short-lived transient species is assigned to bound polaron pairs, and the long-lived transient species is assigned to positive and negative polarons.

Wavelength-dependent photochemistry in Cr(CNPh)6: a study of photosubstitution and photoinduced electron transfer using time-resolved spectroscopy.

The wavelength dependence of photosubstitution, photoinduced electron transfer, and the time-resolved spectra of Cr(CNPh)6, a compound having low-lying MLCT states, were investigated. Photosubstitution quantum yields increase with increasing excitation energy while photoinduced electron transfer quantum yields decrease with increasing excitation energy. At the lowest excitation energy used (532 nm, or 18,800 cm(-1)), the quantum yields for both electron transfer and photosubstitution reach the same maximum value, 0.29. Picosecond time-resolved absorption spectra at 355 and 532 nm excitation wavelengths show two features: a bleach signal centered at 400 nm and an excited state absorption (ESA) in the 600 nm region. The ESA signal is much weaker for 532 nm excitations than for 355 nm excitations. Following a 355 nm flash, the bleach and ESA decay exponentially with the same lifetime of 23 micros. This implies a simple ligand dissociation followed by recombination. Bleach recovery kinetics after a 532 nm flash are more complicated: two or three exponential components are required to fit the data. Cr(CNPh)6 exhibits two photochemical mechanisms: at high excitation energy, a simple charge neutral dissociation occurs; at low energy, it is proposed that a phenylisocyanide radical anion dissociates, forming a radical pair that is responsible for the observed substitution and electron transfer reactivity, and the complicated nanosecond kinetics. The primary processes for both reactions occur in less than 20 ps.

Investigation of the primary electron transfer processes in some rigidly bridged dyads and triads by (sub)picosecond pump–probe experiments

The electron transfer kinetics of several completely rigid dyads and triads which contain N,N-dimethylaniline (DMA) and dimethoxynaphthalene (DMN) as possible donors and the dicyanovinyl group (DCV) as acceptor were studied by means of (sub)picosecond time-resolved transient absorption spectroscopy. In the dyads DMN[n]DCV, the rate of charge separation decreases exponentially with the number of σ-bonds n in the bridge, the ‘‘damping factor’’ being 0.8 per bond in tetrahydrofuran solvent. In the triads DMA[4]DMN[8]DCV, the primary electron transfer from DMN to DCV occurs in solvents of low and medium polarity within 10 ps in both isomers (syn and anti). The rates of the secondary electron transfer step (formation of the fully charge separated state, DMA+[4]DMN[8]DCV−) and the following deactivation processes depend, however, strongly on the conformation. In acetonitrile, the primary electron transfer involves the two donor groups yielding preferentially DMA+[4]DMN−[8]DCV. In the anti-conformer this state is fairly long-lived; in the syn-conformer, however, it decays rapidly, in part to locally excited triplet states.

Picosecond time-resolved absorption spectroscopy of luciferin

We present the results of picosecond time-resolved absorption spectroscopy of luciferin, substrate of the bioluminescence reaction. Both absorption and emission from the excited states contribute to the detected signal. Time evolution of the differential optical density at different wavelengths is analyzed. Characteristic rate constants of the relaxation processes are determined. The possible relationship between the behavior of the luciferin molecule in the excited state and the properties of the luciferin–luciferase complex is discussed.

Self-Assembly of Zincporphyrin Dimer and Pyromellitimide Using Two Coordination Bonds and Photoinduced Intramolecular Electron Transfer

Abstract Zincporphyrin dimer-pyromellitimide supramolecule has been designed and assembled using two coordination bonds. Photoinduced charge separation and charge recombination in the supramolecule were observed by picosecond time-resolved transient absorption measurements.

Photoinduced intramolecular charge-transfer systems based on porphyrin–viologen dyads for optical limiting

Photoinduced intramolecular charge transfer in porphyrin–viologen dyads has been investigated as a potential method leading to optical limiting in the nanosecond range. Solutions of these dyads in acetonitrile were studied by picosecond and nanosecond time-resolved fluorescence and absorption spectroscopy. Both methods give evidence of efficient photoinduced charge separation. Nonlinear transmission measurements around 600 nm with nanosecond laser pulses have shown that the dyad solution leads to a lower transmission at high incident fluences (>1 J/cm 2 ) than the model porphyrin solution. Moreover, this dyad solution is more efficient than a solution of porphyrin with added viologen, both solutions having the same quenching efficiency.

Interfacial Carriers Dynamics of CdS Nanoparticles

The relaxation dynamics of charge carriers in 4 nm CdS colloidal quantum dots are studied by means of picosecond time-resolved fluorescence and femtosecond transient absorption experiments. We also studied the effects of the adsorption of viologen derivatives as electron acceptors on the surface of these particles. From these experimental measurements, we reached a model of the electron-hole dynamics in these nanoparticles consistent with previous proposals. In particular, we have confirmed that the electron trapping in these particles is slower than the hole trapping (30 ps versus a few picoseconds). After excitation, rapid formation of an optical hole (bleach) within the lowest energy exciton (band gap) absorption region appears. The maximum of the bleaching band is red-shifted by 20 meV in 2.5 ps, and the bleach intensity recovers in 30 ps. Upon the adsorption of electron acceptors, the rate of the red shift of the optical hole is not affected while the bleach recovery time is reduced to a few picoseco...

Vibrational Relaxation and Redistribution in the 2Ag- State of all-trans-Lycopene As Revealed by Picosecond Time-Resolved Absorption Spectroscopy

Vibrational relaxation in the 2Ag- (S1) state of all-trans-lycopene, which takes place in a time scale slower than that of internal conversion, has been found by picosecond time-resolved absorption spectroscopy: The time-resolved Sn (2Bu+) ← S1 (2Ag-) absorption spectra of this carotenoid in quinoline and CS2 solutions can be explained in terms of vibrational structures originating from the CC (ν1) and C−C (ν2) stretching modes, and the time scales of vibrational relaxation (kν1 and kν2 defined for the lowest vibrational transition) relative to that of internal conversion (k) are estimated by spectral simulation; the values of kν1/k and kν2/k are around 0.6 and 0.3 in the quinoline solution and 0.2 and 1.6 in the CS2 solution. The slower vibrational relaxations are discussed in light of the light-harvesting function of carotenoids in photosynthesis.

Picosecond time-resolved dual fluorescence, transient absorption and reorientation time measurements of push-pull diphenyl-polyenes: evidence for ‘loose’ complex and ‘bicimer’ species

Picosecond streak camera measurements, time-resolved absorption and reorientation time measurements (by optical Kerr effect) of push-pull diphenyl-polenes (4-dimethylamino 4′-cyano stilbene: DCS, 1-(p-N,N-dimethylamino)-4-(p-cyanophenyl)-1,3-butadiene: DCB) in solution have revealed, in polar solvents at high concentration and high excitation intensity, the presence of a new emitting species characterized by an anomalous red-shifted fluorescence band. The formation rate and decay excited state decay time of the new species is measured for the first time at room temperature as well as the spectral characteristics of the fluorescence. The intensity of the new fluorescence band has been found to be proportional to the square of the excitation intensity. Furthermore, the reorientation time at high concentration has been determined to be nearly twice as long as that at low concentration, indicating the possibility of the formation of complexes in the ground state with nearly twice the molecular volume compared with the monomer. This bimolecular and biphotonic process, demonstrated by the experimental results, makes it possible to reveal a ‘loose’ complex formation in the ground state, leading upon high excitation intensity to the new emitting species already called ‘bicimer’ in preliminary studies and formed by the association of two electronically excited molecules.

Synthesis and Photoinduced Electron Transfer Processes in Ru(II)(bpy)2/Os(III)(bpy)2-Based Triad Complexes Containing Functionalized Diimide Ligands

Abstract The charge separated(CS) state was efficiently formed as a result of stepwise electron transfer reactions in a novel Ru(II)(bpy)2-diimide-Os(III)(bpy)2 triad system (efficiency > 0.7); the rate of electron transfer and the charge separation yield were determined from picosecond time-resolved absorption spectra.

Deactivation of excited 4′-dialkylamino-9-styrylacridines

The absorption and emission spectra and deactivation of excited styrylacridines are determined by the flexibility and charge transfer properties of the donor-acceptor chromophore system. The fluorescence and excitation spectra, as well as the kinetics, vary with the excitation and emission wavelength respectively. Principal component analysis (PCA) and global analysis of the spectra and kinetics of the E isomers reveal at least two absorbing and two emitting components in fluid solution at room temperature as well as in a frozen matrix. At 77 K, the fluorescence of the Z isomer is also observed. The S n ← S 1 transition of the Z isomer is the only transient detectable from picosecond time-resolved absorption experiments in solution. The main competing deactivation channels of the S 1 state are E → Z isomerization, charge transfer-induced internal conversion and fluorescence.

Triplet exciton formation in a benzophenone single crystal studied by picosecond time-resolved absorption spectroscopy

The formation of triplet excitons in a benzophenone single crystal has been studied by picosecond time-resolved absorption spectroscopy. The rate constant of the singlet-triplet intersystem crossing has been determined to be (28 ± 2 ps) −1. With strong excitation the formation of the triplet exciton becomes faster. This is ascribed to the fission of a highly excited state, populated by a biphotonic process, into two triplet excitons.

Time-resolved spectroscopy of wild-type and mutant Green Fluorescent Proteins reveals excited state deprotonation consistent with fluorophore-protein interactions

Recently steady-state and picosecond time-resolved absorption and fluorescence spectroscopy on the Green Fluorescent Protein (GFP) have been interpreted by a mechanism where the key process is an excited state deprotonation of the chromophore (M. Chattoraij, B.A. King, G.U. Bublitz and S.G. Boxer, Proc. Natl. Acad. Sci. USA, 93 (1996) 8362–8367). Such a conclusion was borne out by the mirror image of the picosecond decay of the protonated species RH ∗ in the blue and the concomitant picosecond rise of the green fluorescence of the deprotonated fluorophore R −∗ as well as the significant slowing of both kinetic features upon deuteration. We report similar experiments confirming this mechanism. The results of ultrafast spectroscopy on wild-type GFP together with two important mutants combined with the recent crystal structures are shown to shed more light on the interplay between absorption and emission phenomena in GFP. Beyond some differences with previous results pertaining, for instance, to the assignment of vibronic progressions in absorption spectra and the temperature dependence of excited state deprotonation, several new features have been identified. These concern the deprotonated ground state R − in equilibrium as well as the excited state RH ∗. In particular, we have studied the distributed fluorescence kinetics in the time and frequency domain, excited state absorption features observed in femtosecond time-resolution, and the dependence of excited state proton transfer kinetics on the aggregational state of the protein.

The small reorganization energy of C 60 in electron transfer

Porphyrin-linked fullerene 1 and porphyrin-linked quinone 2 have been designed and synthesized to evaluate the size and shape effect of fullerene in electron transfer. For evaluation of such an effect C 60 and benzoquinone with comparable reduction potentials have been chosen and linked with a porphyrin, where a relatively rigid spacer ensures similar separation distance and nature of the inteervening bonds between the redox pair. Accelerated charge separation and deceleratd charge recombination were observed in 1 compared with 2 by picosecond fluorescence lifetime measurements and time-resolved transient absorption spectroscopy. They may be explained by the smaller reorganization energy in C 60 compared with those in small acceptors.